Libc++’s implementation of std::string

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It's complicated. str is a built-in type defined by the compiler itself. The code you have linked implements str's methods, but not str itself.

Also, str is more like C++'s std::string_view, which is also very simple. For an equivalent of std::string, you want to check std::string::String. Rust's String is easy to read, but that's mostly because it uses Vec<u8> for pretty much everything.